Websites which target globally many countries have usually a dropdown in the home page to select language & country. Sometimes they are configured to point visitors to localized pages by redirects or through changing the content to the user’s language. rel-alternate-hreflang annotation is a new way supported by Both Google & Yandex to specify homepages. This would look like
<link rel=”alternate” href=”http://website.com/” hreflang=”x-default” />
<link rel=”alternate” href=”http://website.com/en-us” hreflang=”en-us” /> Targetted for US
The new x-default hreflang attribute value tells the Google algorithm that website.com is the default page & doesn’t target any country or language. This helps webmasters set x-default hreflang attribute value to suit particular requirements of the country based websites. This can be done through sitemaps or through HTML coding. More info about the same can be had at official Google Webmaster Central blog